Bike generators are powered by people. People biking require time. What is the labor or opportunity cost of having all of these people sustaining 250W per hour or essentially a .25kW-hour? Just a thought but that doesn't sound cheap.
That's exactly what jsilence is saying. Energy from the grid is so incredibly cheap that you're using it without a second thought on things you would never consider doing without a grid. It could go up hundredfold before becoming expensive.
